Thermally addressed cholesteric-smectic liquid crystal device

ABSTRACT

A visual display device is featured which uses a new thermal addressing technique to provide a dark image upon a lighter background. The display is capable of being multiplexed to a large number of rows. The device comprises a cholesteric-smectic liquid crystal material mixed with a pleochroic dye of high order parameter. When the material experiences a transition from a higher to a lower, smectic thermal phase, two different textures or light states are developed; a transparent state and a light absorbing state. The transparent state is developed by applying a sensitizing voltage to certain portions of the medium. The pleochroic dye absorbs the light passing through the unsensitized portions of medium to provide the dark image. The row electrodes are made diffusely reflective so as to provide a double light pass through the medium to improve contrast.

FIELD OF INVENTION

The invention pertains to thermally addressable cholesteric-smecticliquid crystal display devices, and more particularly to thermallyaddressed visual display devices which use a light absorption techniqueto provide a dark image upon a lighter background.

BACKGROUND OF THE INVENTION

Heretofore, the ability to fabricate large scale multiplexed liquidcrystal displays was very difficult. This difficulty was primarily dueto "cross-talk" effects, and the necessity to quickly refresh the slowresponding liquid crystal medium. Large scale multiplexed displaysnotoriously have had problems with "cross-talk", i.e., the unwantedsensitizing of partially selected display elements. This problem resultsfrom the small root mean square voltage ratio between the "on" and "off"elements achievable in a large scale multiplexed liquid crystal display.

As displays become larger, a new problem appears. Most device effects donot have intrinsic storage. The display must therefore be repeatedlyscanned to update; this is often with typical display effects done at 60Hz (per frame). The result for large area matrices is a small duty cyclefor each individual row or column. Most display media only partiallyrespond to small duty cycle voltage information and the resulting effectis only a fraction of the dc equivalent voltage. The result is lowcontrast or brightness. As the display matrix gets larger, the dutycycle gets less and less and optical performance gets poorer and poorer.The result is a very poor (below commercial standards) opticalperformance as the X-Y matrix gets larger and larger.

These two problems have severely limited the ability to provide largescale multiplexed displays, and to date, no one has produced a devicewhich has high contrast, wide viewing angle, which is easy to fabricate,easy to operate, and which has low cost.

The present invention has developed a low cost, large scale multiplexed,visual display device that has resolved the aforementioned problems,while providing a new liquid crystal device having many advantages overthe prior art.

While the present invention is concerned primarily with large scale,thermally addressed multiplexed devices, its new light absorbing methodis easily applicable to devices which are not large scale, and which donot utilize multiplexing. The subject invention is believed to have wideapplication in the field of thermally addressed liquid crystal displays,and is not considered as being limited to any particular device orsystem.

DISCUSSION OF RELATED ART

The invention features certain classes of smectic liquid crystal hoststhat have a cholesteric phase upon heating. A small percentage ofpleochroic dye is added to the material. The display is addressed in athermal electric mode. For the "on" elements, the liquid crystal textureis light absorbing due to the dye which strongly absorbs incoming light.The "off" elements and the background have hometropic smectric Atexture, where the dye exhibits minimum absorption.

The concept of pleochroic dye switching as the Guest Host effect innematic liquid crystals, was first suggested in an article to: G. H.Heilmeier, J. A. Castellano, and L. A. Zanoni, Mol. Crystals and LiquidCrystals 8, 293 (1969).

Others have suggested that the liquid crystal structure can be twistednematic, homogeneous, or homeotropic. Most of these devices usingpleochroic dyes mixed with the liquid crystal material have generallyrequired external devices such as polarizers or wave plates to improvethe contrast of the image.

Dyes of high order parameter in a cholesteric liquid crystal host werefirst suggested in an article to: D. L. White, G. N. Taylor, J. of AppPhys. 45 4718 (1974).

Displays using this liquid crystal medium have high contrast and do notrequire external polarizers. These displays have high brightness and awide viewing angle not available with the field effect twisted nematicliquid crystal displays. Such devices use a cholesteric to nematictransition effect with liquid crystal displays. Such devices use acholesteric to nematic transition effect with liquid crystals ofpositive dielectric anisotrophy.

In the no field (off) mode, the dye molecules follow the helicalstructure of the host and exhibit strong light absorption. In the oncondition, the dye is in a homeotropic nematic host and the absorptionis minimized. Thus, the display presents a white image against a dark(or colored) background. A white image against a dark background is,however, generally not desirable. In addition, it has been well reportedthat such a cholesteric to nematic transition effect cannot bemultiplexed above approximately 5-10 lines and give commercialperformance. This is due to the change in the slope of the contrastversus voltage relationship that causes "cross-talk".

Recently, a paper was presented in the 8th International Liquid CrystalConference at Kyoto, Japan by Professor A. Sasaki et al., entitled"Laser Addressed Liquid Crystal Multifunction Light Valve"; in which, hedescribed a laser addressed projection display utilizing a liquidcrystal of 90:10 mixture of p-p' cyano-octyl biphenyl and cholesterylnonanoate. The mixture should have a cholesteric phase followed by asmectic A phase upon cooling. However, the display is a projectiondevice that derives its image contrast purely through scattering. Thethermal addressing is by a scanning laser beam. No dyes are used in hismaterial.

Recently, high order parameter and light stable dyes have becomeavailable. Devices using these dyes will provide viable displays formany applications. However, they have two major drawbacks which mayrestrict their application to simple displays of very low informationcontent only.

These dye displays are very difficult to multiplex. Even a few rowsrepresent a state of the art development. Large size matrix addressinghas only been achieved by adding external non-linear elements to eachdisplay element.

For non-emissive (reflective) displays, a white image against a darkbackground is formed. This is aesthetically undesirable and of limitedcommercial utility. Techniques to reverse the image contrast to a morepleasing dark against a light background are available, but the addedcomplication increases the complexity and cost.

In 1978 C. Tani and T. Ueno discussed the application of pleochroic dyesto certain smectic liquid crystals in a scientific paper (Appl. Phys.Lett. Vol. 33 No. 4, Aug. 15, 1978). The authors, however, specificallyteach against the use of the smectic "A" phase as having utility in thepleochroic dye system: they indicate that it has application only inscattering applications such as in laser addressed light valves. Theyconcluded that only materials having smectic H or possibly B phasestructure have useful properties in combination with pleochroic dyes.Further, they discuss the utilization of slow cooling as having utilitywith pleochroic dyes and that rapid cooling of the elements is onlyapplicable to light scattering devices.

The present invention utilizes pleochroic dyes to produce an absorbingstate rather than a scattering state and uses thermal XY local heatingas distinct from the laser heating as described in other art. Further itutilizes rapid cooling of the element with liquid crystalspreferentially of the smectic "A" phase. The last factor is directlyagainst the teaching of Tani and Ueno, but has been found to be mosteffective in this application.

Also recently, a system has been reported in the French Literature,which uses a thermally addressed smectic "A" crystal medium. Such asystem is desribed in an article entitled: MATRIX ADDRESSED SMECTICLIQUID CRYSTAL DISPLAY: M. Hareng, S. Le Berre, R. Hehlen, and J. N.Perbet, Thomson-CSF Laboratoire Central de Rechereches. Proceedings fromSociety of Information Display 1980 Conference, Late News Paper.

Such a system does not use dyes, and employs a scattering lighttechnique, rather than a light absorption technique as described by thisinvention.

In addition, the system described is embodied in a very different devicethan detailed in this invention. Because of the crucial difference ofthe light scattering as compared to light absorption, the device canonly be viewed through a projection optical system that results in avery bulky, power intensive system.

While the prior art teaches the use of pleochroic dyes of high orderparameter for use in liquid crystal media, it should also be noted thatthese dyes are used primarily to enhance the light affects produced bythe thermal phase transition of the media. The invention by contrast,relies upon the dye to do most of the light absorption for the crystalmedium; the medium acting as a vehicle for orientating the dye todevelop a light absorbing stance.

BRIEF DESCRIPTION OF THE INVENTION

The invention relates to a thermally addressed visual device whichprovides a dark image against a lighter background. The device comprisesa liquid crystal medium including at least one cholesteric liquidcrystal compound mixed with at least one coloring agent, generally apleochroic dye of high order parameter. The medium has positivedielectric anisotropy. The medium is thermally sensitive and has atransition between at least two thermal phases; the upper thermal phaseis a cholesteric phase and a lower thermal phase is a smectic phase. Themedium develops two textures in the smectic phase: a light absorbingtexture and a homeotropic texture. The homeotropic texture is developedin portions of the medium by sensitizing the medium as it passes rapidlyfrom the upper cholesteric phase to the lower, smectic phase. The lightabsorbing texture develops in the unsensitized portions of the medium asit goes through the transition of the smectic phase.

The medium is sensitized by applying a voltage to those portions of themedium to be addressed. The addressed portions develop a substantiallytransparent light state, while the unaddressed portions develop asubstantially light absorbing state. The coloring agent or dye which islocked within the liquid crystal medium as it develops its lightabsorbing texture in the smectic phase will absorb most of the lightpassing through the medium; the liquid crystal acting as a vehicle toorient the dye molecules into a light absorbing position. Electrodes areprovided to sensitize the medium. They are disposed adjacent the medium.Heating electrodes are also provided to heat the medium to an upperthermal phase. in a multiplexed device, these electrodes define a matrixof columns and rows disposed substantially at right angles to eachother, and in different planes.

In order to obtain a pleasing direct viewable display, the rowelectrodes are made diffusely reflective to provide high contrast aswell as wide viewing angle. The reflective electrodes provide for adouble pass of light through the cell enhancing light absorbing.

The liquid crystal medium will generally contain an alkyl cyano biphenylcompound and will generally have two thermal transitions; between anisotropic and cholesteric phase, and between the cholesteric and asmectic "A" phase.

Many liquid crystal compounds with optically active terminal groupsexhibit cholesteric phase. Some of them also exhibit one or more smecticphases when the compounds are cooled down from the cholesteric phase.For example, a paper published by Joseph A. Castellano, C. S. Oh, M. T.McCaffray, Mol. Crystal. Liq. Cryst., V. 27 pp 417, 1973, lists 40schiff base compounds with the general structure as: ##STR1##

Many compounds with high value of m and n exhibit a cholesteric phasefollowed by smectic phases upon cooling. To cite a few examples, wehave: ##STR2##

Although these compounds have the desirable phase transitions for deviceapplication, Schiff bases are generally not very stable. Also, there areother requirements that a practical material should have. Thus, typicalworking materials are formalized with stable compounds at suitablecomposition.

One of the requirements for the host liquid crystal is that itsdielectric anisotropy should be strongly positive. This is usuallyobtained by using liquid crystal compounds having C.tbd.N as one of theterminal groups.

One example of a workable cholesteric liquid crystal comprises a mixtureof X, Y and Z materials, each having a percentage by weight in anapproximate range of: 40 to 60 of X; 30 to 50 of Y; and 5 to 15 of Z;respectively, where: ##STR3##

More particularly, the aforementioned mixture can comprise: ##STR4##

Which has a phase transition as follows: ##STR5##

In a liquid crystal which has a smectic phase followed by a nematicphase, good display performance, requires the temperature range of thenematic phase to be narrow. With cholesteric materials, however, thetemperature range of the cholesteric phase does not necessarily have tobe narrow.

To the host material, a high order parameter pleochroic dye or dyemixture is added in a range of approximately 0.5 to 3 percent by weightof the total composition.

More particularly, about 1% by weight of a purple dye having theformula: ##STR6## is added to the above cholesteric liquid crystalmedium. This dye is sold by E. M. Laboratories, Elmsford, N.Y.

In operating the device, the liquid crystal medium is passed rapidlythrough its thermal transition from its upper cholesteric phase to itslower thermal, smectic phase. The transition must be accomplishedreasonably rapidly, hence rapid thermal pulses are used that heat theliquid crystal locally but do not significantly heat the surroundingglass. Hence the natural cooling period immediately following thepassage of the heat pulse is also rapid and hence the liquid crystalmedium passes through the nematic phase rapidly. This greatly enhancesthe optical effect and results in greater contrast.

Certain portions of the medium are sensitized. These portions define thebackground of the medium. These sensitized portions develop thesubstantially light transparent state when the medium passes into thesmectic thermal phase. The remaining unsensitized portions of the mediumdevelop a light absorbing state. When light (generally ambient) ispassed through the medium, the unsensitized portions absorb the light toprovide a dark image upon the lighter sensitized background. Theaddressed portions of the medium may be sensitized in a chronologicalsequence.

When the liquid crystal material cools down either from the isotropicstate through the cholesteric state to the smectic state, or fromcholesteric to smectic state, the texture obtained in the smectic statedepends on the cooling rate, surface alignment, the pitch of thecholesteric molecules and some other factors. The materials bestsuitable for this new device have molecular pitch in the 1-3 μm region.Most display devices have perpendicular alignment on both glasssurfaces. This type of alignment is not absolutely necessary for thisnew display.

When the cooling rate is slow (for example less than 500° C./min) wehave two cases:

(1) Cooling from the isotropic phase through a narrow (approximately 10°C. or less) cholesteric phase: a clear homeotropic texture is obtained.

(2) Cooling from the cholesteric phase to smectic phase: a scatteringS_(A) texture is obtained if the cooling rate is up to 100° C./min.region. With slower cooling rate, the clear homeotropic texture isobtained.

With fast cooling rate corresponding to the actual display operation (upto 250,000° C./min.), scattering textures are always obtained.

The scattering state obtained with fast cooling in the cholesteric tosmectic system has finer structure as compared to those obtained withnematic to smectic system. When a pleochroic dye is added to thematerial, the scattering state becomes a light absorbing state. Becauseof its finer structure, the color is very deep. It is an object of thisinvention to provide improved liquid crystal device, method, andcholesteric liquid crystal compositions;

It is another object of the invention to provide an improved largescale, multiplexed, thermally addressed visual display;

It is a further object of this invention to provide a highly contrasteddark image on a lighter background for a thermally addressed cholestericliquid crystal device.

These and other objects of the invention will become more apparent andwill be better understood with reference to the following detaileddescription considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective, exploded, schematic view of a visual devicemade in accordance with the invention;

FIG. 2 is a plan schematic view of the device shown in FIG. 1,illustrating how an image can be formed in the cholesteric-smecticliquid crystal medium by a multiplexing technique;

FIG. 3 is a graphical illustration of the chronological sequencing ofthe row and column electric waveforms of the device depicted in FIG. 1.

FIGS. 4a and 4b show a schematic view of two different light modulatingtextures developed in the cholesteric liquid crystal medium of thedevice of FIG. 1, when the medium passes rapidly into its smectic phaseFIG. 4a depicts a homeotropic, substantially light transparent texture,and FIG. 4b illustrates a substantially light absorbing texture.

DETAILED DESCRIPTION OF THE INVENTION

Generally speaking, this invention relates to new methods, compositions,and visual devices utilizing the thermal addressing ofcholesteric-smectic liquid crystal media. The visual devices of thisinvention feature a highly contrasted dark image on a lighterbackground.

Where the devices of the invention are multiplexed, they are capable ofbeing multiplexed up to a large number of rows.

This invention provides new displays that incorporate pleochroic dyes ofhigh order parameter into a smectic A liquid crystal material that has acholesteric phase upon heating. By using a thermal electric addressingtechnique described hereinafter, this display has major advantages overthe previously know dye switching displays.

A cholesteric liquid crystal with positive dielectric anisotropy candevelop a homeotropic texture under the influence of an electric field.A homeotropic smectic A phase is formed, if the material is rapidlycooled through the phase transition. The homeotropic S_(A) phase isclear or transparent and shows very little color (colorless) withdissolved pleochroic dye. Without an electric field, a light absorbingtexture is formed in the medium. Thus, by controlling the electric fieldacross the liquid crystal layer during the cholesteric to Smectic Aphase transition, one can create at his will, either a colored state ora non-colored state. Once these states are formed, they are stable untilerased by heating into the isotropic or cholesteric phase again.

Although the above description assumes that the material is heated intothe isotropic state, it is noted that this is not absolutely necessary.In reality, only heating to the cholesteric state is needed. Also, dueto the different physical mechanisms of forming the colored scatteringstate, the temperature range of the cholesteric state does notnecessarily have to be narrow to ensure a good display performance.

The smectic A phase can be aligned homeotropically, as shown in FIG. 4a,if the surface of the display is treated with materials such asLecithin. In this structure, the material is transparent.

There are two forms of thermally addressed smectic A displays. One typeuses a scanning laser beam to address the display elements. The othertype is x y matrix addressed. The row electrodes are heated sequentiallywith electric current and the display is written by applying voltages onthe columns. During the writing process, only the dots associated to therow where the heating current has just been removed are affected. Inother words, only the dots where the liquid crystal material is coolingto the smectic state respond to the writing pulses on the columnelectrodes.

As the liquid crystal material cools rapidly through the cholestericphase to the smectic phase, it can form two different textures. With avoltage applied on the column, the liquid crystal material is switchedto a homeotropic state during the cholesteric phase and assumes thetransparent homeotropic smectic A texture after cooling is completed.Without the applied voltage, a light absorbing texture is developedinstead. Thus, the dots associated with a cooling row electrode can bewritten into a transparent state or a light absorbing state by applyingor not applying voltages on the columns. The cholesteric-smecticmaterial used in the invention display device has positive dielectricanisotropy. The transition must be accomplished reasonably rapidly,hence rapid thermal pulses are used that heat the liquid crystal locallybut do not significantly heat the surrounding glass. Hence the naturalcooling period immediately following the passage of the heat pulse isalso rapid and hence the liquid crystal medium passes through thecholesteric phase rapidly. This greatly enhances the optical effect andresults in greater contrast.

The present invention, however, must be carefully distinguished fromother similar systems wherein a scattering texture rather than a lightabsorbing texture is developed in the smectic material. Displaysdeveloping the scattering texture are generally not suitable for directviewing, and are often used only in projection systems.

The optical contrast developed by a scattering texture against atransparent texture is similar to those obtained with the dynamicscattering effect. Under many commonly encountered illuminationconditions, it will not give a pleasing, high contrast image.

The situation becomes quite different, however, when a pleochroic dye ofhigh order parameter is introduced into the smectic A material. The dyebecomes locked into the liquid crystal, and assumes the orientation ofthe liquid crystal molecules. The dye molecules in the scatteringtexture of the host absorb light strongly, transforming the normalscattering texture into a light absorbing texture, either deeply coloredor dark, as shown in FIG. 4b. In the homeotropic smectic texture, thedye molecules have minimum absorption, since they do not absorb lightincident upon the edge of their molecular structure. This texture,therefore, develops a transparent background. This results in a highcontrast display that is suitable for direct viewing. No externalpolarizers are required. The addressing technique is substantially thesame as smectic displays without the dye.

Now referring to FIG. 1, an exploded view of a typical multiplexed,visual display device 10, is illustrated. The device comprises acholesteric-smectic liquid crystal medium 11 containing the pleochroicdye, which material is disposed between two glass substrate plates 12and 13, respectively. The top substrate plate 12 supports a plurality ofcolumn electrodes C₁, C₂, C₃, etc., which make up one half of the x ymatrix for addressing the liquid crystal material 11. The columnelectrodes are made of electrically conductive, light transparentmaterial such as indium tin oxide, which can be vacuum deposited on theglass plate 12.

The bottom plate 13 supports a plurality of row electrodes r₁, r₂, r₃,etc., which make up the remaining half of the x y matrix. The rowelectrodes are electrically conductive and are made diffusely reflectivewith material such as silver or aluminum. The row electrodes aredesigned to be diffusely reflective in order to provide good displayimage with wide viewing angle.

The liquid crystal medium 11 is generally sealed between the twosubstrate plates 12 and 13 with the electrodes in contact on eitherside. Light (generally ambient) is passed through (arrow 18) the glasscomposite, as shown.

The physical operation of this display 10 can best be illustrated with asimple example of a 5×7 matrix displaying a character "A", as shown inFIG. 2. The rows of the matrix are tied together at one end to thecommon 16 and are sequentially heated by applying electric pulses to theother ends 17. In time zone 0, (see FIG. 3) row 1 is heated such thatthe liquid crystal material over the row 1 electrode r₁ is in theisotropic or cholesteric state. In time zone 1, row 2 electrode r₂ isheated. In the meantime, row 1 rapidly cools down and the dotsassociated with it are written by applying electric voltage on thecolumn electrodes. In this example, electrodes C₁ and C₅ have voltageapplied such that the dots r₁ c₁ and r₁ c₅ will be in the transparentstate. C₂, C₃, C₄ have no voltage applied, and the dots r₁ c₂, r₁ c₃, r₁c₄ have a colored light absorbing texture. During time zone 2, row 3electrode r₃ is heated and row 2 cools down, and the voltage on thecolumns assume the values corresponding to the "on" and "off" pattern ofdots associated to row 2. The entire waveform for displaying a character"A", is shown in FIG. 2.

The colored light absorbing texture associated to the "on" dots ismetastable and has long relaxation time generally over a few months.This texture can be automatically erased by heating the row duringrewriting of the display. The light absorbing texture is not effected bythe writing voltage applied on the column electrodes. This assures that"cross-talk" will not be a problem, and makes possible a large scalematrix display.

The erase-writing process for this display is very fast. Generally, lessthan a 100μ second writing time can be achieved. If the display isrefreshed at f_(R) times per second, the total number of rows that canbe multiplexed will be

    M=1/(f.sub.R ×T)

where T=the time required to write the row.

With f_(R) =30 hertz, which is a similar rate as a conventional CRT, andT=100 m sec., we have n=333 rows. Thus, the display can be multiplexedup to a rather large number of rows.

In practical display driving, the heating pulse can be applied overseveral time zones before the cooling and writing cycle. This lowers thevoltage requirement for the heating pulses. However, the heating pulseshould be short enough to avoid heat spreading to the neighboring rowsand to minimize glass heating that inhibits rapid cooling.

A high contrast is achieved for the colored or black image due to thelight absorbing character of the dye material vis-a-vis the transparentbackground.

The contrast is further improved by the diffusely reflective nature ofthe row electrodes, which provide a double light pass back through(arrow 15) the medium 11, wherein the unaddressed dye molecules in thelight absorbing state (image) can absorb more light as compared to theaddressed transparent background.

The medium 11 is depicted in the sensitized (addressed) homeotropicphase in FIG. 4a, and is shown in the unaddressed light absorbing phasein FIG. 4b. Light (arrow 20) entering the homeotropic material of FIG.4a, passes between the liquid crystal molecules 21. The dye molecules 22are not light absorbing in this phase, because they are locked in thecrystal to confront the light rays upon their edge, as shown.

However, in the light absorbing phase, the dye molecules 22 are lockedin the crystal molecules 21 in a randomly angled pattern, as shown inFIG. 4b. In this phase, the dye molecules 22 will strongly absorb theimpinging light rays 20 to produce an intensely colored or dark image.

The crystal liquid medium 11 can be comprised of at least one alkylcyano biphenyl compound.

More particularly, the liquid crystal will be comprised of a mixture ofcyano biphenyl compounds of the following formulas: ##STR7##

One example of a workable cholesteric liquid crystal comprises a mixtureof X, Y, and Z materials, each having a percentage by weight in anapproximate range of: 40 to 60 of X; 30 to 50 of Y' and 5 to 15 of Z;respectively, where: ##STR8##

More particularly, the aforementioned mixture can comprise: ##STR9##

Which has a phase transition as follows: ##STR10##

In a liquid crystal which has a smectic phase followed by a nematicphase, good display performance, requires the temperature range of thenematic phase to be narrow. With cholesteric materials, however, thetemperature range of the cholesteric phase does not necessarily have tobe narrow.

To the host material, a high order parameter pleochroic dye or dyemixture is added in a range of approximately 0.5 to 3 percent by weightof the total composition.

More particularly, about 1% by weight of a purple dye having theformula: ##STR11## is added to the above cholesteric liquid crystalmedium. This dye is sold by E. M. Laboratories, Elmsford, N.Y.

While the medium generally features pleochroic dyes of high orderparameter, it is also contemplated that other coloring agents such as:##STR12## may also provide reasonable image contrast.

Having thus described the invention, what is desired to be protected byLetters Patent is presented by the following appended claims.

What is claimed is:
 1. A thermally addressed visual device whichprovides a dark image against a lighter background comprising:A liquidcrystal medium including at least one liquid crystal cholestericcompound mixed with at least one pleochroic dye of high order parameterand having dielectric anisotropy, said medium being thermally sensitiveand having a transition between at least two thermal phases, an upperthermal phase being a cholesteric phase and a lower thermal phase beinga smectic phase, said medium having two possible light modulating statesin said smectic phase, a first light state being normally substantiallylight absorbing and a second light state being substantiallytransparent; and means to apply a sensitizing voltage to addressportions of said medium to develop said second light transparent statewhen said medium passes rapidly into said smectic phase, the remainingportions of said medium developing said first substantially lightabsorbing state.
 2. The thermally addressed visual device of claim 1,wherein said pleochroic dye is light stable.
 3. The thermally addressedvisual device of claim 1, wherein at least one liquid crystal compoundcomprises a smectic "A" liquid crystal material in said lower phase. 4.The thermally addressed visual device of claim 1, wherein said liquidcrystal medium comprises an alkyl cyano biphenyl.
 5. The thermallyaddressed visual device of claim 1, wherein said medium passes rapidlythrough two transitions between isotropic and cholesteric phases, andrapidly between cholesteric and smectic "A" phases.
 6. The thermallyaddressed visual device of claim 1, wherein said medium passes rapidlythrough two transitions between isotropic and cholesteric phases, andrapidly between cholesteric and smectic "A" phases, and furthercomprising means for passing said medium between a lower and an upperphase including at least one heating electrode disposed adjacent saidmedium.
 7. The thermally addressed visual device of claim 6, whereinsaid heating electrodes are disposed adjacent said medium and diffuselyreflect light passing through said medium back through said medium. 8.The thermally addressed visual device of claim 1, wherein said means forsensitizing portions of said medium comprises at least one electrode forapplying a sensitizing voltage to said addressed portions of saidmedium.
 9. The thermally addressed visual device of claim 1, whereinsaid liquid crystal medium comprises a mixture of at least two liquidcrystal compounds.
 10. A thermally addressed visual device whichprovides a dark image against a lighter background, said dark imagebeing observable from the same side as a light source passing lightthrough said device, comprising:a liquid crystal medium including atleast one liquid crystal cholesteric compound mixed with at least onecoloring agent and having dielectric anisotropy, said medium beingthermally sensitive and having a transition between at least two thermalphases, an upper thermal phase being a cholesteric phase and a lowerthermal phase being a smectic phase, said medium having two possiblelight modulating states in said smectic phase, a first light state beingnormally substantially light absorbing and a second light state beingsubstantially transparent; means disposed adjacent said medium fordiffusely reflecting light passing through said medium back through saidmedium; and means to apply a sensitizing voltage to address portions ofsaid medium to develop said second light transparent state when saidmedium passes rapidly into said smectic phase, the remaining portions ofsaid medium developing said first substantially light absorbing state.11. The thermally addressed visual device of claim 10, wherein at leastone coloring agent comprises a pleochroic dye.
 12. The thermallyaddressed visual device of claim 11, wherein said pleochroic dye is ofhigh order parameter.
 13. The thermally addressed visual device of claim11, wherein said pleochroic dye is light stable.
 14. The thermallyaddressed visual device of claim 10, wherein at least one liquid crystalcompound comprises a smectic "A" liquid crystal material in said lowerphase.
 15. The thermally addressed visual device of claim 10, whereinsaid liquid crystal medium comprises an alkyl cyano biphenyl.
 16. Thethermally addressed visual device of claim 10, wherein said mediumpasses rapidly through two transitions between isotropic and cholestericphases, and between cholesteric and smectic "A" phases.
 17. Thethermally addressed visual device of claim 10, wherein said mediumpasses rapidly through two transitions between isotropic and cholestericphases, and between cholesteric and smectic "A" phases, and furthercomprising means for passing said medium between a lower and an upperphase including at least one heating electrode disposed adjacent saidmedium.
 18. The thermally addressed visual device of claim 17, whereinsaid heating electrodes are disposed adjacent said medium and diffuselyreflect light passing through said medium back through said medium. 19.The thermally addressed visual device of claim 10, wherein said meansfor sensitizing portions of said medium comprises at least one electrodefor applying a sensitizing voltage to said portions of said medium. 20.The thermally addressed visual device of claim 10, wherein said meansfor diffusely reflecting said light comprises at least one lightdiffusing reflective electrode.
 21. The thermally addressed visualdevice of claim 10, wherein said means for diffusely reflecting saidlight and said means for heating said medium are comprised in at leastone diffusely reflective heating electrode.
 22. The thermally addressedvisual device of claim 10, wherein said liquid crystal medium comprisesa mixture of at least two liquid crystal compounds.
 23. A thermallyaddressed visual device which provides a dark image against a lighterbackground, comprising:a liquid crystal medium including at least oneliquid crystal cholesteric compound mixed with at least one pleochroicdye of high order parameter and having dielectric anisotropy, saidmedium being thermally sensitive and having a transition between anupper and a lower thermal phase, said upper thermal phase being acholesteric phase and said lower thermal phase being a smectic phase,said medium having two possible light modulating states in said smecticphase characterized by development of respective light absorbing andhomeotropic textures in said medium, said light absorbing texture beingnormally substantially light absorbing and said homeotropic texturebeing substantially transparent, and means to apply a sensitizingvoltage to address portions of said medium to develop said lighttransparent texture when said medium passes rapidly into said smecticphase, said unaddressed portions of said medium developing said lightabsorbing texture wherein said dye absorbs light passing through saidunaddressed portions of said medium.
 24. The thermally addressed visualdevice of claim 23, wherein said pleochroic dye is light stable.
 25. Thethermally addressed visual device of claim 23, wherein at least oneliquid crystal compound comprises a smectic "A" liquid crystal materialin said lower phase.
 26. The thermally addressed visual device of claim23, wherein said liquid crystal medium comprises an alkyl cyanobiphenyl.
 27. The thermally addressed visual device of claim 23, whereinsaid medium passes rapidly through two transitions between isotropic andcholesteric phases, and rapidly between cholesteric and smectic "A"phases.
 28. The thermally addressed visual device of claim 23, whereinsaid medium passes rapidly through two transitions between isotropic andcholesteric phases, and rapidly between cholesteric and smectic "A"phases, and further comprising means for passing said medium between alower and an upper phase including at least one heating electrodedisposed adjacent said medium.
 29. The thermally addressed visual deviceof claim 23, wherein said means for sensitizing portions of said mediumcomprises at least one electrode for applying a sensitizing voltage tosaid addressed portions of said medium.
 30. The thermally addressedvisual device of claim 23, wherein said heating electrodes are disposedadjacent said medium and diffusely reflect light passing through saidmedium back through said medium.
 31. The thermally addressed visualdevice of claim 23, wherein said liquid crystal medium comprises amixture of at least two liquid crystal compounds.
 32. A thermallyaddressed visual device which provides a dark image against a lighterbackground, comprising:a liquid crystal medium including at least oneliquid crystal cholesteric compound mixed with at least one coloringagent and having dielectric anisotropy, said medium being thermallysensitive and having a transition between an upper and a lower thermalphase, said upper thermal phase being a cholesteric phase and said lowerthermal phase being a smectic phase, said medium having two possiblelight modulating states in said smectic phase, characterized bydevelopment of respective light absorbing and homeotropic textures insaid medium, said light absorbing texture being normally substantiallylight absorbing and said homeotropic texture being substantiallytransparent; means disposed adjacent said medium for diffuselyreflecting light passing through said medium back through said medium;and means to apply a sensitizing voltage to address portions of saidmedium to develop said homeotropic texture when said medium passesrapidly into said smectic phase, said unaddressed portions of saidmedium developing said light absorbing texture wherein said coloringagent absorbs light passing through said unaddressed portions of saidmedium.
 33. The thermally addressed visual device of claim 32, whereinat least one coloring agent comprises a pleochroic dye.
 34. Thethermally addressed visual device of claim 33, wherein said pleochroicdye is of high order parameter.
 35. The thermally addressed visualdevice of claim 33, wherein said pleochroic dye is light stable.
 36. Thethermally addressed visual device of claim 32, wherein at least oneliquid crystal compound comprises a smectic "A" liquid crystal materialin said lower phase.
 37. The thermally addressed visual device of claim32, wherein said liquid crystal medium comprises an alkyl cyanobiphenyl.
 38. The thermally addressed visual device of claim 32, whereinsaid medium passes rapidly through two transitions between isotropic andcholesteric phases, and rapidly between cholesteric and smectic "A"phases.
 39. The thermally addressed visual device of claim 32, whereinsaid medium passes rapidly through two transitions between isotropic andcholesteric phases, and rapidly between cholesteric and smectic "A"phases, and further comprising means for passing said medium between alower and an upper phase including at least one heating electrodedisposed adjacent said medium.
 40. The thermally addressed visual deviceof claim 39, wherein said heating electrodes are disposed adjacent saidmedium and diffusely reflect light passing through said medium backthrough said medium.
 41. The thermally addressed visual device of claim32, wherein said means for sensitizing portions of said medium comprisesat least one electrode for applying a sensitizing voltage to saidportions of said medium.
 42. The thermally addressed visual device ofclaim 32, wherein said means for diffusely reflecting said lightcomprises at least one light diffusing reflective electrode.
 43. Thethermally addressed visual device of claim 32, wherein said means fordiffusely reflecting said light and said means for heating said mediumare comprised in at least one diffusely reflective heating electrode.44. The thermally addressed visual device of claim 32, wherein saidliquid crystal medium comprises a mixture of at least two liquid crystalcompounds.
 45. A thermally addressed visual device which provides a darkimage against a lighter background, said dark image being observablefrom the same side as a light source passing light through said device,comprising:a liquid crystal medium including at least one liquid crystalcholesteric compound mixed with at least one pleochroic dye of highorder parameter and having dielectric anisotropy, said medium beingthermally sensitive and having a transition between an upper and a lowerthermal phase, said upper thermal phase being a cholesteric phase andsaid lower thermal phase being a smectic phase, said medium having twopossible light modulating states in said smectic phase, characterized bydevelopment of respective light absorbing and heomeotropic textures insaid medium, said light absorbing texture being normally substantiallylight absorbing and said homeotropic texture being substantiallytransparent; means for passing said medium through said transitionbetween said upper and said lower phase; and means to apply asensitizing voltage to address portions of said medium to develop saidhomeotropic texture when said medium passes rapidly into said smecticphase, said unaddressed portions of said medium developing said lightabsorbing texture wherein said dye absorbs light passing through saidunaddressed portions of said medium.
 46. The thermally addressed visualdevice of claim 45, wherein said pleochroic dye is light stable.
 47. Thethermally addressed visual device of claim 45, wherein at least oneliquid crystal compound comprises a smectic "A" liquid crystal materialin said lower phase.
 48. The thermal addressed visual device of claim45, wherein said liquid crystal medium comprises an alkyl cyanobiphenyl.
 49. The thermally addressed visual device of claim 45, whereinsaid medium passes rapidly through two transitions between isotropic andcholesteric phases, and rapidly between cholesteric and smectic "A"phases.
 50. The thermally addressed visual device of claim 45, whereinsaid medium passes rapidly through two transitions between isotropic andcholesteric phases, and rapidly between cholesteric and smectic "A"phases, and said means for passing said medium between a lower and anupper phase comprises at least one heating electrode disposed adjacentsaid medium.
 51. The thermally addressed visual device of claim 50,wherein said heating electrodes are disposed adjacent said medium anddiffusely reflect light passing through said medium back through saidmedium.
 52. The thermally addressed visual device of claim 45, whereinsaid means for sensitizing portions of said medium comprises at leastone electrode for applying a sensitizing voltage to said addressedportions of said medium.
 53. The thermally addressed visual device ofclaim 45, wherein said liquid crystal medium comprises a mixture of atleast two liquid crystal compounds.
 54. The thermally addressed visualdevice of claim 45, wherein said means for passing said medium throughsaid thermal transition comprises at least one electrode.
 55. Athermally addressed visual device which provides a dark image against alighter background, said dark image being observable from the same sideas a light source passing light through said device, comprising:A liquidcrystal medium including at least one liquid crystal cholestericcompound mixed with at least one coloring agent and having dielectricanisotropy, said medium being thermally sensitive and having atransition between an upper and a lower thermal phase, said upperthermal phase being a cholesteric phase and said lower thermal phasebeing a smectic phase, said medium having two possible light modulatingstates in said smectic phase characterized by development of respectivelight absorbing and homeotropic textures in said medium, said lightabsorbing texture being normally substantially light absorbing and saidhomeotropic texture being substantially transparent; means disposedadjacent said medium for diffusely reflecting light passing through saidmedium back through said medium; and means to apply a sensitizingvoltage to address portions of said medium to develop said homeotropictexture when said medium passes rapidly into said smectic thermal phase,said unaddressed portions of said medium developing said light absorbingtexture wherein said coloring agent absorbs light passing through saidunaddressed portions of said medium.
 56. The thermally addressed visualdevice of claim 55, wherein at least one coloring agent comprises apleochroic dye.
 57. The thermally addressed visual device of claim 56,wherein said pleochroic dye is of high order, parameter.
 58. Thethermally addressed visual device of claim 56, wherein said pleochroicdye is light stable.
 59. The thermally addressed visual device of claim55, wherein at least one liquid crystal compound comprises a smectic "A"liquid crystal material in said lower phase.
 60. The thermally addressedvisual device of claim 55, wherein said liquid crystal medium comprisesan alkyl cyano biphenyl.
 61. The thermally addressed visual device ofclaim 55, wherein said medium passes rapidly through two transitionsbetween isotropic and cholesteric phases, and rapidly betweencholesteric and smectic "A" phases.
 62. The thermally addressed visualdevice of claim 55, wherein said medium passes rapidly through twotransitions between isotropic and cholesteric phases, and rapidlybetween cholesteric and smectic "A" phases, and further comprising meansfor passing said medium between a lower and an upper phase including atleast one heating electrode disposed adjacent said medium.
 63. Thethermally addressed visual device of claim 62, wherein said heatingelectrodes are disposed adjacent said medium and diffusely reflect lightpassing through said medium back through said medium.
 64. The thermallyaddressed visual device of claim 55, wherein said means for sensitizingportions of said medium comprises at least one electrode for applying asensitizing voltage to said addressed portions of said medium.
 65. Thethermally addressed visual device of claim 55, wherein said means fordiffusely reflecting said light comprises at least one light diffusingreflective electrode.
 66. The thermally addressed visual device of claim55, wherein said means for diffusely reflecting said light and saidmeans for heating said medium are comprised in at least one diffuselyreflective heating electrode.
 67. The thermally addressed visual deviceof claim 55, wherein said liquid crystal medium comprises a mixture ofat least two liquid crystal compounds.
 68. A thermally addressed visualdevice which provides a dark image against a lighter background, saiddark image being observable from the same side as a light source passinglight through said device, comprising:a liquid crystal medium includingat least one liquid crystal cholesteric compound mixed with at least onecoloring agent and having dielectric anisotropy, said medium beingthermally sensitive and having a transition between an upper and a lowerthermal phase, said upper thermal phase being a cholesteric phase andsaid lower thermal phase being a smectic phase, said medium having twopossible light modulating states in said smectic phase, characterized bydevelopment of respective light absorbing and homeotropic textures insaid medium, said light absorbing texture being normally substantiallylight absorbing and said homeotropic texture being substantiallytransparent; means for passing said medium through said transitionbetween said cholesteric and said smectic phase; means disposed adjacentsaid medium for diffusely reflecting light passing through said mediumback through said medium; and means to apply a sensitizing voltage toaddress portions of said medium to develop said homeotropic texture whensaid medium passes rapidly into said smectic phase, said unaddressedportions of said medium developing said light absorbing texture whereinsaid coloring agent absorbs light passing through said unaddressedportions of said medium.
 69. The thermally addressed visual device ofclaim 68, wherein at least one coloring agent comprises a pleochroicdye.
 70. The thermally addressed visual device of claim 69, wherein saidpleochroic dye is of high order, parameter.
 71. The thermally addressedvisual device of claim 69, wherein said pleochroic dye is light stable.72. The thermally addressed visual device of claim 68, wherein at leastone liquid crystal compound comprises a smectic "A" liquid crystalmaterial in said lower phase.
 73. The thermally addressed visual deviceof claim 68, wherein said liquid crystal medium comprises an alkyl cyanobiphenyl.
 74. The thermally addressed visual device of claim 68, whereinsaid medium passes rapidly through two transitions between isotropic andcholesteric phases, and rapidly between cholesteric and smectic "A"phases.
 75. The thermally addressed visual device of claim 68, whereinsaid medium passes rapidly through two transitions between isotropic andcholesteric phases, and rapidly between cholesteric and smectic "A"phases, and said means for passing said medium between an upper and alower phase comprises at least one heating electrode disposed adjacentsaid medium.
 76. The thermally addressed visual device of claim 75,wherein said heating electrodes are disposed adjacent said medium anddiffusely reflect light passing through said medium back through saidmedium.
 77. The thermally addressed visual device of claim 68, whereinsaid means for sensitizing portions of said medium comprises at leastone electrode for applying a sensitizing voltage to said portions ofsaid medium.
 78. The thermally addressed visual device of claim 68,wherein said means for diffusely reflecting said light comprises atleast one light diffusing reflective electrode.
 79. The thermallyaddressed visual device of claim 68, wherein said means for diffuselyreflecting said light and said means for heating said medium arecomprised in at least one diffusely reflective heating electrode. 80.The thermally addressed visual device of claim 68, wherein said liquidcrystal medium comprises a mixture of at least two liquid crystalcompounds.
 81. The thermally addressed visual device of claim 68,wherein said means for passing said medium through said thermaltransition comprises at least one electrode.
 82. A thermally addressed,multiplexed, visual device which provides a dark image against a lighterbackground, said dark image being observable from the same side as alight source passing light through said device, comprising:a liquidcrystal medium including at least one liquid crystal cholestericcompound mixed with at least one coloring agent and having dielectricanisotropy, said medium being thermally sensitive and having atransition between an upper and a lower thermal phase, said upperthermal phase being a cholesteric phase and said lower thermal phasebeing a smectic phase, said medium having two possible light modulatingstates in said smectic phase, characterized by development of respectivehomeotropic and light absorbing textures in said medium, a first lightstate being normally substantially light absorbing and a second lightstate being substantially transparent; means for passing said mediumthrough said transition between said cholesteric and said smectic phase;means disposed adjacent said medium for diffusely reflecting lightpassing through said medium back through said medium; and means tosequentially apply sensitizing voltages to address portions of saidmedium to develop said second substantially light transparent state whensaid medium passes rapidly into said smectic phase, said mediumdeveloping said homeotropic texture in said addressed portions, theremaining unaddressed portions of said medium developing said firstsubstantially light absorbing state, said unaddressed portions of saidmedium developing said light absorbing texture wherein said coloringagent absorbs light passing through said unaddressed portions of saidmedium.
 83. The thermally addressed visual device of claim 82, whereinat least one coloring agent comprises a pleochroic dye.
 84. Thethermally addressed visual device of claim 83, wherein said pleochroicdye is of high order, parameter.
 85. The thermally addressed visualdevice of claim 83, wherein said pleochoric dye is light stable.
 86. Thethermally addressed visual device of claim 82, wherein at least oneliquid crystal compound comprises a smectic "A" liquid crystal materialin said lower phase.
 87. The thermally addressed visual device of claim82, wherein said liquid crystal medium comprises an alkyl cyanobiphenyl.
 88. The thermally addressed visual device of claim 82, whereinsaid medium passes rapidly through two transitions between isotropic andcholesteric phases, and rapidly between cholesteric and smectic "A"phases.
 89. The thermally addressed visual device of claim 82, whereinsaid medium passes rapidly through two transitions between isotropic andcholesteric phases, and rapidly between cholesteric and smectic "A"phases, and said means for passing said medium between a lower and anupper phase comprises at least one heating electrode disposed adjacentsaid medium.
 90. The thermally addressed visual device of claim 89,wherein said heating electrodes are disposed adjacent said medium anddiffusely reflect light passing through said medium back through saidmedium.
 91. The thermally addressed visual device of claim 82, whereinsaid means for sensitizing portions of said medium comprises at leastone electrode for applying a sensitizing voltage to said portions ofsaid medium.
 92. The thermally addressed visual device of claim 82,wherein said means for diffusely reflecting said light comprises atleast one light diffusing reflective electrode.
 93. The thermallyaddressed visual device of claim 82, wherein said means for diffuselyreflecting said light and said means for heating said medium arecomprised in at least one diffusely reflective heating electrode. 94.The thermally addressed visual device of claim 82, wherein said liquidcrystal medium comprises a mixture of at least two liquid crystalcompounds.
 95. The thermally addressed visual device of claim 82,wherein said means for passing said medium through said thermaltransition comprises at least one electrode.
 96. A thermally addressed,multiplexed, visual display device which provides a dark image against alighter background comprising:A liquid crystal medium including at leastone liquid crystal cholesteric compound mixed with at least onepleochoric dye of high order parameter and having dielectric anisotropy,said medium being thermally sensitive and having a transition between atleast two thermal phases, said medium having two possible lightmodulating states in a lower thermal phase, an upper thermal phase beinga cholesteric phase and said lower thermal phase being a smectic phase,a first light state being normally substantially light absorbing and asecond light state being substantially transparent; and a matrix ofelectrodes disposed about said medium, a number of said matrixelectrodes defining rows and a number of said matrix electrodes definingcolumns, said row electrodes heating said medium to cause said medium topass through a thermal transition between said lower and said upperphase, and said column electrodes for applying sequential sensitizingvoltages to address portions of said medium to develop said second lighttransparent state when said medium rapidly cools and passes into saidsmectic phase, the remaining unaddressed portions of said mediumdeveloping said first substantially light absorbing state.
 97. Thethermally addressed visual device of claim 96, wherein said pleochroicdye is light stable.
 98. The thermally addressed visual device of claim96, wherein at least one liquid crystal compound comprises a smectic "A"liquid crystal material in said lower phase.
 99. The thermally addressedvisual device of claim 96, wherein said liquid crystal medium comprisesan alkyl cyano biphenyl.
 100. The thermally addressed visual device ofclaim 96, wherein said medium passes rapidly through two transitionsbetween isotropic and cholesteric phases, and rapidly betweencholesteric and smectic "A" phases.
 101. The thermally addressed visualdevice of claim 96, wherein said medium passes rapidly through twotransitions between isotropic and cholesteric phases, and rapidlybetween cholesteric and smectic "A" phases, and wherein said electrodesare disposed adjacent said medium.
 102. A thermally addressed,multiplexed, visual display device providing a dark image against alighter background, comprising:A liquid crystal medium including atleast one liquid crystal cholesteric compound mixed with at least onepleochroic dye of high order parameter and having dielectric anisotropy,said medium being thermally sensitive and having a transistion betweenat least two thermal phases, said medium having two possible lightmodulating states in a lower thermal phase, an upper thermal phase beinga cholesteric phase and said lower thermal phase being a smectic phase,a first light state being normally substantially light absorbing and asecond light state being substantially transparent; and a matrix ofelectrodes disposed about said medium, a number of said matrixelectrodes defining rows and a number of said matrix electrodes definingcolumns, said column electrodes being substantially at right angles to,and disposed in another plane with respect to said row electrodes, saidrow electrodes for heating said medium to cause said medium to passrapidly through a thermal transition between said lower and said upperphase, and said column electrodes for applying sequential sensitizingvoltages to address portions of said medium to develop said second lighttransparent state when said medium cools and passes into said smecticphase, the remaining unaddressed portions of said medium developing saidfirst substantially light absorbing state.
 103. The thermally addressedvisual device of claim 102, wherein said pleochroic dye is light stable.104. The thermally addressed visual device of claim 102, wherein atleast one liquid crystal compound comprises a smectic "A" liquid crystalmaterial in said lower phase.
 105. The thermally addressed visual deviceof claim 102, wherein said liquid crystal medium comprises an alkylcyano biphenyl.
 106. The thermally addressed visual device of claim 102,wherein said medium passes rapidly through two transitions betweenisotropic and cholesteric phases, and rapidly between cholesteric andsmectic "A" phases.
 107. The thermally addressed visual device of claim102, wherein said medium passes rapidly through two transitions betweenisotropic and cholesteric phases, and rapidly between cholesteric andsmectic "A" phases, and wherein said rows are disposed adjacent saidmedium.
 108. A thermally addressed, multiplexed, visual display deviceproviding a dark image against a lighter background, said image beingobservable from the same side as a light source passing light throughsaid device, comprising:A liquid crystal medium including at least oneliquid crystal cholesteric compound mixed with at least one coloringagent and having dielectric anisotropy, said medium being thermallysensitive and having a transition between at least two thermal phases,said medium having two possible light modulating states in a lowerthermal phase, an upper thermal phase being a cholesteric phase and saidlower thermal phase being a smectic phase, a first light state beingnormally substantially light absorbing and a second light state beingsubstantially transparent; and a matrix of electrodes disposed aboutsaid medium, a number of said matrix electrodes defining rows and anumber of said matrix electrodes defining columns, said columnelectrodes being substantially transparent to light passing through saidmedium, said row electrodes being substantially diffusely reflective forheating said medium to cause said medium to pass through a thermaltransistion between said lower and said upper phases, said columnelectrodes for applying sequential sensitizing voltages to addressportions of said medium to develop said second light transparent statewhen said medium rapidly cools and passes into said smectic phase, theremaining portions of said medium developing said first substantiallylight absorbing state, said row electrodes being disposed behind saidmedium and having means to diffusely reflect the light passing throughthe medium back through said medium.
 109. The thermally addressed visualdevice of claim 108, wherein at least one coloring agent comprises apleochroic dye.
 110. The thermally addressed visual device of claim 109,wherein said pleochroic dye is of high order parameter.
 111. Thethermally addressed visual device of claim 109, wherein said pleochroicdye is light stable.
 112. The thermally addressed visual device of claim108, wherein at least one liquid crystal compound comprises a smectic"A" liquid crystal material in said lower phase.
 113. The thermallyaddressed visual device of claim 108, wherein said liquid crystal mediumcomprises an alkyl cyano biphenyl.
 114. The thermally addressed visualdevice of claim 108, wherein said medium passes rapidly through twotransitions between isotropic and cholesteric phases, and rapidlybetween cholesteric and smectic "A" phases.
 115. In a thermallyaddressed visual display device having a liquid crystal medium includingat least one liquid crystal cholesteric compound mixed with at least onecoloring agent, the method of displaying a dark image against a lighterbackground, comprising the steps of:(a) passing said medium rapidlythrough a thermal transition from an upper thermal, cholesteric phase toa lower thermal, smectic phase; (b) sensitizing certain portions of saidmedium defining a background to develop a substantially lighttransparent state in said background when said medium passes rapidlyinto said lower thermal, smectic phase, the remaining unsensitizedportions of said medium developing a light absorbing state; (c) passinglight through said medium; and (d) absorbing light in said unsensitizedportions of said medium to provide a dark image upon said lightersensitized background.
 116. The method of claim 115, wherein said darkimage is observable from the same side as the light passing through saidmedium, and comprising the further step of:(e) diffusely reflecting thelight passing through said medium back through said medium.
 117. Themethod of claim 115, wherein said medium has dielectric anisotropy andfurther wherein the sensitized portions of said medium are sensitized bythe further step of:(e) applying a voltage to said certain portions ofsaid medium in order to sensitize said certain portions into developingsaid light transparent state.
 118. The method of claim 117, wherein saidvoltage is applied in a given chronological sequence to said certainportions of said medium.
 119. The method of claim 115, wherein saidsensitized portions of said medium are sensitized in chronologicalsequence.
 120. The method of claim 115, further comprising the stepof:(e) passing said medium through said transition from said lowerthermal, smectic phase to said upper thermal, cholesteric phase prior tostep (a).
 121. The method of claim 120, wherein said transition step (e)is accomplished by heating said medium.
 122. The method of claim 115,wherein said transition step (a) is accomplished by rapidly cooling saidmedium.
 123. The method of claim 115, wherein said medium passes rapidlythrough two thermal transitions rapidly between an upper isotropicphase, an intermediate cholesteric phase, and a lower smectic phase.124. The method of claim 115, wherein said light passing through saidmedium in step (c) comprises ambient light.
 125. In a thermallyaddressed visual display device having a liquid crystal medium includingat least one liquid crystal cholesteric compound mixed with at least onepleochroic dye of high order parameter and having dielectric anisotropy,said medium being thermally sensitive and having a transition between anupper and a lower thermal phase, said upper thermal phase being acholesteric phase and said lower thermal phase being a smectic phase,said medium having two possible light modulating states in said lowerthermal, smectic phase characterized by development of respective lightabsorbing and homeotropic textures in said medium, said light absorbingtexture being normally substantially light absorbing and saidhomeotropic texture being substantially transparent, the method ofdisplaying a dark image against a lighter background, comprising thesteps of:(a) rapidly passing said medium through said transition fromsaid upper thermal, cholesteric phase to said lower thermal, smecticphase; (b) sensitizing certain portions of said medium defining abackground to develop said second substantially light transparent statein said background when said medium passes rapidly into its lowerthermal, smectic phase, the remaining unsensitized portions of saidmedium developing said light absorbing state; (c) passing light throughsaid medium; and (d) absorbing light in said unsensitized portions ofsaid medium to provide a dark image upon said lighter sensitizedbackground.
 126. The method of claim 125, wherein said dark image isobservable from the same side as the light passing through said medium,and comprising the further step of:(e) diffusely reflecting the lightpassing through said medium back through said medium.
 127. The method ofclaim 126, wherein the sensitized portions of said medium are sensitizedby the further step of:(e) applying a voltage to said certain portionsof said medium in order to sensitize said certain portions intodeveloping said light transparent state.
 128. The method of claim 127,wherein said voltage is applied in a given chronological sequence tosaid certain portions of said medium.
 129. The method of claim 126,wherein said sensitized portions of said medium are sensitized inchronological sequence.
 130. The method of claim 126, further comprisingthe step of:(e) passing said medium through said transition from saidlower thermal, smectic phase to said upper thermal, cholesteric phaseprior to step (a).
 131. The method of claim 130, wherein said transitionstep (e) is accomplished by heating said medium.
 132. The method ofclaim 126, wherein said transition step (a) is accomplished by rapidlycooling said medium.
 133. The method of claim 126, wherein said mediumpasses rapidly through two thermal transitions between an upperisotropic phase, an intermediate cholesteric phase, and a lower smecticphase.
 134. The method of claim 126, wherein said light passing throughsaid medium in step (c) comprises ambient light.